1. Field of the Invention
The present invention relates to a thermal transfer recording method for transferring the ink contained in an ink sheet onto a recording medium thereby recording an image thereon, and an apparatus adapted for effecting said method.
The above-mentioned thermal transfer recording apparatus includes a facsimile apparatus, an electronic typewriter, a copying machine, a printer or the like.
2. Related Background Art
In general, the image recording in a thermal transfer printer is achieved by utilizing an ink sheet formed by coating a base film with a heat-fusible (or heat-sublimable) ink, selectively heating said ink sheet corresponding to image signal with a thermal head and transferring thus fused (or sublimed) ink onto a recording sheet. Said ink sheet is usually a so-called one-time ink sheet which completely loses the ink after an image recording, so that it is necessary, after the recording of a character or a line, to advance the ink sheet by an amount corresponding to said recording, in order to insure that the unused portion of the ink sheet is brought to the next recording position. This fact increases the amount of use of the ink sheet, so that the running cost of a thermal transfer printer tends to be higher than that of the ordinary thermal printer in which the recording is made on thermal recording paper.
In order to solve this drawback, a thermal transfer printer in which the recording sheet and the ink sheet are advanced with different speeds is proposed for example in U.S. Pat. No. 4,456,392, the Japanese Laid-open Patent Sho 58-201686 and the Japanese Patent Publication Sho 62-58917. Also described in these patent references is the known so-called multi print sheet, which is an ink sheet capable of plural image recordings, and, in continuous recording of a length L, such multi print sheet allows the amount of advancement of the ink sheet, during or after the image recording to be less than said length L (L/n:n&gt;1). Such method improves the efficiency of use of the ink sheet to n times, so that a reduction in the running cost of the thermal transfer printer can be expected. This method is hereinafter called the multi-printing method.
The present inventors have experimentally confirmed that the multi-printing with the thermal transfer method is preferably conducted with a larger relative speed between the recording sheet and the ink sheet, as will be explained in the following.
In the conventional heat transfer method, the ink of the ink sheet, fused by a heating, has to be completely peeled from the base film. However, in the multi-printing method in which the ink sheet is repeatedly used in n times, about 1/n of the ink layer has to be peeled and transferred by each heating. On the other hand, since the ink layer of the ink sheet is heat-fusible, there is required a larger shearing force for separating the ink layer, as the time from the heating with the thermal head to the peeling of the ink layer increases. It will therefore become difficult to properly peel the ink layer and to transfer the same onto the recording sheet (by 1/n) when said time becomes longer. Thus the separation of the ink layer by 1/n may not be properly conducted unless the relative speed between the recording sheet and the ink sheet is maintained at a certain level.
This drawback may particularly become a problem in a recording apparatus with intermittent advancement of the recording sheet, such as a facsimile apparatus. Let us consider a facsimile apparatus, as shown in FIG. 18, having a recording line length of 1/15.4 mm and having a thermal head which is divided into four blocks driven in succession with an interval of 2.5 ms. The shearing force for separating the ink of the ink layer heated by a block of the thermal head, from the other part of the ink layer, appears at the energization of the first of the blocks A-D (T'.sub.E represents the timing of energization), namely when the recording sheet is conveyed by a line in response to a command to start recording for the next line. However, at the energization of 2nd to 4th blocks, the recording sheet and the ink sheet are stopped together, so that the time until the generation of the necessary shearing force becomes longer. Since said time fluctuates in random manner for example in the facsimile apparatus, the shearing force for separating the ink layer also shows fluctuation.
In order to securely peel the ink layer of the ink sheet and transfer the same onto the recording sheet, copying with thus increased shearing force, there are required high-torque motors for conveying the recording sheet and the ink sheet. Because such motors are expensive, the cost of the entire apparatus is inevitably increased. Also since the recording sheet is generally conveyed by the friction with a platen roller, the shearing force exceeding a certain level may result in improper transportation of the recording sheet or the transporting speed of the recording sheet being affected by that of the ink sheet